1. Field of the Invention
The present invention relates to a paper sheet conveyance method of a post processing apparatus to handle an image-formed paper sheet, especially, a post processing apparatus for a center-center-folded sheet.
2. Description of the Related Art
In the past, a post processing apparatus functioning to fold or bind paper sheets, which is designed to hook up to an image forming apparatus such as a copier, and an image forming apparatus equipped with the post processing apparatus have been developed and folding processes or technologies to eject stacked and bound paper sheets have been disclosed. (For example, FIG. 5, Page 4 of Laid Open Japanese Patent Application No. 2001-151406 and 2003-182928)
An ejecting configuration for paper sheets disclosed in Laid-Open Japanese Patent Publication No. 2001-151406 is that a folded portion of paper sheet in a valley side of center-folded sheets (dual-folded) contacts a leading edge portion of a paper sheet-supporting device and is loaded thereon, and the paper sheet-supporting device moves and transfers the center-folded sheets to paired rollers provided above the supporting deice, when a number of a paper sheet reaches a predetermined number. Then the center-folded sheets are unloaded. The paired rollers also has a function to load the center-folded sheets, which have been folded by a paper sheet-folding device, to the paper sheet supporting device by rotating the rollers reversibly, which is a feature to minimize the size of apparatus and simplifies the configuration of the apparatus.
In Laid Open Japanese Patent Publication No. 2003-182928, provided is, post processing apparatus, which forms a folded-line at the center of the sheet, by nipping the center of the sheet to a pair of first rollers while pressing the center of the sheet by a holding plate member, and re-creasing the folded portion by a pair of second rollers, which are arranged to move perpendicular to the paper conveyance direction when pressing the folded portion while in motion, when folding a thick bundle of large number of paper sheets.
A book binder having an endless chain to run more than one clamp to handle tied bundles of paper sheets, which greatly improves productivity has been disclosed in, for example, Laid Open Japanese Patent Publication No. 10-35135.
In recent years, customer's demands have increased greatly for a folding process capable of a small to a large number of paper sheets or binding process. However, configurations disclosed in Laid Open Japanese Patent Publication No. 2001-151406 and Laid Open Japanese Patent Publication No. 2003-182928 are likely to exhibit problem such as inferior conveyance and crinkles occurring in the paper sheets. This tends to occur, in case of configurations having a pair of rollers to eject a bound paper sheets. Conveyance force is applied onto the outer paper sheets of the bound of paper sheets and enough force cannot be transferred across the bound paper sheets due to frictions between the paper sheets similarly, even the configuration disclosed in Laid Open Japanese Patent Publication No. 10-35135 exhibits problem is that crinkles are likely to occur at the outer paper sheets of a bundle of such sheets.
In addition, once binding pressure of a pair of rollers or clamps to transfer forces to be applied to all paper sheets of a bundle of such sheets, conveyance force necessary for a bundle of small number of paper sheets cannot be produced. This results in the bundle of paper sheets dropping down while being conveyed or inferior conveyance such as inclination of the bundles of paper sheets, and the post processing apparatus becomes a large structure since binding pressure control equipment is required to function for a bundle of a few paper sheets to one consisting of a large number of paper sheets.
Up to now, a post processing apparatus having a cutting device to trim the edges of the booklets, which have been bound in the middle and/or, folded in the printing industry.
Furthermore, in recent years, copy machines, printers, etc. have been offered, featuring a cutting device, which trims bound sheet edges for booklets such as a weekly magazine, after a center binding process and folding process have been affected.
A cutting device disclosed in registered Utility model No. 2516010 is one which cuts the edges of sheets the booklets after pressing a folded portion by a first pressing member and after a predetermined time pressing adjacent edge portion of the booklet by a second pressing member.
In conventional post processing devices, a too far turned booklet caused by conveyance is corrected by continued rotation of upper and lower conveyer belts (being called overrun-time) for a constant time period after the booklet on which center-binding and center folding processes had been carried out, is conveyed until the leading edge of the booklet in a conveyance direction hits a stopper, as the booklet is sandwiched between the lower conveyance driving belt and the upper following conveyance belt.
In recent years, high productivity bookbinder including post process is required. When shortening of the aforementioned overrun-time in order to satisfy that request, there is a case that a too far turn of the booklet cannot be corrected.
Other weak points were such as occurrence of crinkles and adhesion of foreign matter depending on the variety of paper sheets on which the overrun occurred, since exceeded conveyance turning of a booklet is corrected while the booklet is sliding down between the upper and lower conveyer belts.
Further, another weak point was the possibility that the cover sheet and inner paper sheets are not aligned because the cover sheet as the outer sheet contacts the upper and lower conveyer belts and a the bulky portion of the booklet shifts in the folded direction when a pressure plate presses the booklet.